In a strain wave gearing, a flexible externally toothed gear is flexed to a non-circular shape, e.g., to an ellipsoidal shape by a wave generator, and caused to partially engage with a rigid internally toothed gear. If the engagement between the internally toothed gear and the externally toothed gear is such that the engagement PCDs of the two gears overlap (engage in an overlapping state), the teeth engagement interference causes, inter alia, a rapid increase in the startup torque and the tooth surface load, significantly degrading the performance of the strain wave gearing and making assembly of the gears difficult. Therefore, the gears in a strain wave gearing are designed to engage in an underlapped state, in which the engagement PCDs of the two gears are slightly apart from each other.
In this underlap design, as shall be apparent, gaps are produced between the teeth of the engaging gears in the vicinity of the long axis of the externally toothed gear flexed to an ellipsoidal shape, and the gaps appear as teeth backlash. In a cup-profile strain wave gearing, the externally toothed gear has a cupped shape; therefore, a preload acts on the opening side in the tooth width direction in the teeth engagement parts near the long axis, and the preload inhibits, to an extent, generation of engagement backlash. In contrast, the engagement backlash in a flat-profile strain wave gearing in which no preload applies is greater than that in a cup-profile strain wave gearing.
Patent Document 1 proposes a reduction gear in which backlash can be reduced. In this reduction gear, elastically deformable external teeth are incorporated and caused to engage with internal teeth in a preloaded state, whereby backlash is reduced.